Submerged arc welding (SAW) is a welding method characterized by high productivity and quality, often used for longer welding seams in thicker materials.
It is well known to use a consumable electrode to conduct a weld current through a work piece. The weld current forms an arc between the consumable electrode and the work piece to create a weld puddle on the work piece. This consumable electrode is known as a hot wire.
Submerged arc welding is characterized in that the melted material and the arcs are protected beneath a layer of pulverized flux. The flux melts in part during the process, thus creating a protecting layer of slag on the weld puddle. The electrical current used in the process is relatively high, usually within 300-1500 Ampere per electrode. The electrodes used in submerged arc welding are usually 2.5- 6 mm in diameter.
Fluxes used in submerged arc welding are granular fusible minerals typically containing oxides of manganese, silicon, titanium, aluminium, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld puddle to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as ‘active’ if they add manganese and silicon to the weld. The amount of manganese and silicon added is influenced by the arc voltage and the welding current level.
It is desirable to increase the productivity of a SAW process. This can be done by increasing the weld speed and the deposition rate, i.e. the rate at which weld metal is actually deposited onto the work piece surface. The heat input should be kept on a level that preserves the mechanical properties of the welded parent material and the weld should have mechanical properties of a certain level.
Heat input can be calculated as follows:
                              Q          =                                    η              SAW                        ⁢                                          I                ×                U                ×                60                            v                        ⁢                          10                              -                3                                                    ,                            (        1        )            
Where Q(kJ/mm) is heat input, ηSAW is an efficiency factor, I(A) is welding current, U(V) is arc voltage and ν(mm/min) is welding speed.
One way to increase the deposition rate is to use multiple hot wires in a single weld puddle. Usually 2-3 hot wires are used, however, usage of up to 6 hot wires is known. Using more than one hot wire in a single weld puddle enables increased deposition rates and therefore improves economy of welding. It also enables improved weld quality due to the possibility of assigning the leading and trailing hot wires with different tasks.
The hot wires can be arranged in various settings or formations. For instance, the hot wires can be positioned shifted out in a transverse direction with respect to a welding direction or be positioned at a distance from each other in the welding direction, or a combination thereof. In the event two or more hot wires are shifted out in the transverse direction, they may be positioned side by side. This is used for surface welding or specific joints where a wide joint is needed. Side by side welding leads to lower penetration and more width.
Alternatively, the hot wires are positioned at a distance from each other in the welding direction. A hot wire located first in the direction of welding is normally referred to as a leading hot wire and a hot wire located behind the leading hot wire is normally referred to as a trailing hot wire. Normally, the leading hot wire and the trailing hot wires serve different roles in the welding process. It is for instance known to control the leading hot wire such that a desired degree of penetration is obtained whereas the trailing hot wire controls weld bead appearance, contour and fill.
It is also possible to separate the hot wires far from one another, in which case the weld puddle generated by the leading hot wire may solidify before a second hot wire has reached the puddle. In this event, the two hot wires more or less perform the task of two consecutive welding passes.
Another way to improve the deposition rate is to add one or more consumable electrodes that melt without formation of arcs. These electrodes are called cold wires. A cold wire is continuously fed towards a molten weld puddle in close proximity to one or more hot wires, where the cold wire is melted by heat generated by said hot wires. A current may be applied to a cold wire for heating thereof.
It is known from, for example, WO 2012/041375 A1 to manipulate the cold wire feed speed independently from the hot wire feed speed. It is also known from JP 2205267 to control the feed speed of a filler wire in dependence on the shape of the groove to maintain a certain melt pool length.
The introduction of cold wire material into the weld puddle may lead to improved control of the composition of the weld alloy, which may lead to improved welds. It is preferable to introduce the cold wire in the vicinity of and preferably into an arc generated by a hot wire (even more preferably in the vicinity of or into arcs generated by a plurality of hot wires). Furthermore, feeding of cold wire material into the weld puddle may lead to an increase of productivity of up to 100% with optimized welding parameters. In other words, a cold wire allows for higher deposition rates without increasing the heat input.
A problem associated with cold wires is that they sometimes increase the welding process instability when the cold wire is not melted at an even pace, or strike the parent material through the melt pool. This can cause weld defects and inclusions in the weld metal of unmelted cold wire material.
A first object of the invention is to provide a method for submerged arc welding using at least one cold wire, which method ensures a stable welding process and improved weld quality.
A second object of the invention is to provide a submerged arc welding system adapted to use at least one cold wire, which system ensures a stable welding process and improved weld quality.